1. Field of the Invention
This invention relates to a circuit arrangement for providing communication channels and power and signalling circuits between a control unit and a terminal. Specifically, the invention relates to such a circuit arrangement in a key telephone system.
2. Description of the Prior Art
There has been a continuing effort on behalf of the telephone industry to reduce the cabling and cross-connecting equipment requirements for key telephone systems. These requirements are based in the desire to allow a key telephone set to be interchangeable, and also for the desire to reduce service assignments at the telephone sets. So called electronic key telephone systems have been proposed in recent years in an effort to reduce the cabling and cross-connecting requirements of the older key telephone systems, and in addition provide features not possible in the prior systems.
For example, in a series of papers published at the 1970 International Conference on Communications, an electronic key telephone system is described in which the control unit and each telephone station set is interconnected by means of six conductors. Two of these six conductors are used for a voice circuit communication channel. The other four conductors are used for data transmission: two conductors for transmission in one direction; the other two conductors for data transmission in the other direction. One paper presented at the 1970 International Conference on Communications entitled "Exploratory Common Control Key Telephone System: System and Functional Description", by L. P. Fabiano, Jr. of Bell Telephone Laboratories, Inc., describes how the six conductor circuit arrangement among four of the six conductors is used to transmit data to and from each station set and how four of the conductors are connected in a phantom pair arrangement for power transmission. Each conductor pair has a center tapped transformer connected both at the control unit and at the station set. Power is supplied from the control unit to the station set over the phantom pair arrangement, while data in one direction is being supplied on one pair of the four conductors and data from the station set to the control unit is being returned over the other pair of the four conductors. The other two leads of the total of six leads, as mentioned above, are used only as a communication channel.
Key telephone systems have been known in which station sets are controlled and powered from a control unit via only four conductors connecting the control unit and each station set. For example, British Pat. No. 1,437,031, assigned to the Plessey Company Limited of Great Britain, describes a four conductor circuit arrangement which connects the control unit for a telephone system to each station set. This British patent describes a "house telephone system" in which the four conductors connecting the control unit to the terminal are divided into two pairs of conductors, one pair being used for power feed and bi-directional signalling, the other pair being used solely as a communication channel or voice circuit path between the station set and the control unit.
The power feed and bi-directional signalling arrangement is described in British Pat. No. 1,437,032, also assigned to the Plessey Company. Power is transmitted from the control unit to the terminal by way of two conductors; simultaneously, data signals, generated in the control unit, are transmitted over the same two conductors to the terminal where they are sensed by a data receiver and applied to the station set. At the station set itself, a series regulator senses the voltage impressed on the two conductors and regulates that voltage in order to supply the terminal station set with d.c. voltage. In series with the series regulator is a constant current source, which is modulated by a current modulator in response to data generated by the terminal. The data generated by the terminal gives information about the characteristics of the terminal itself, and these data are transmitted over the two conductors back to the control unit where they are sensed by a receiver responsive to the current pulses and are used in the terminal to control the terminal itself.
In modern communication systems, such as key telephone systems, there is often a requirement that there be two communication channels connecting the control unit and the terminal. For example, in a key telephone system it is often desirable that a user of a key telephone station set, who is communicating over a first channel, (for example on an outside call over a central office line) be able to simultaneously talk over a second communication channel to someone connected to the key telephone system over an intercom line. Thus, there is a requirement that there be provided not one, but two audio communication channels over the four conductors connecting the control unit and the terminal, with the simultaneous provision of data signalling in both directions (from the control unit to the terminal and from the terminal to the control unit) with simultaneous provision of power being supplied from the control unit to the terminal over the four conductors.
Thus this invention provides a circuit arrangement in which a control unit and a terminal are interconnected by means of only four conductors and in which two communication channels, bi-directional signalling between the control unit and the terminal and power transmission from the control unit to the terminal is provided.
This invention also provides a method for transmitting control data over the four conductor arrangement by which various circuits in a key telephone station set may be controlled and/or monitored. The invention further provides a method of dial pulse signalling simultaneously with control data transmission.